ASoC: arizona: Add signal activity output for DRC

When doing signal activity detection, the only output from the DRC will
often be a GPIO pin. This patch adds a signal activity output that is
activated when a GPIO is configured to output the DRC signal activity
detection.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>